1. Field of the Invention
The invention relates to a process for preparing an aqueous binder dispersion BM, comprising a polyacrylate-modified polyurethane-alkyd resin with a mixture of unsaturated fatty acids as monomer building blocks, and to the use of such binder dispersions BM in coating compositions for low-yellowing, alcohol-resistant, high-gloss coatings, especially for wood and plastic.
2. Prior Art
Aqueous alkyd resin acrylate dispersions for the preparation of air-drying coatings have been known for a relatively long time and are described, for example, in U.S. Pat. No. 4,133 786 or GB-A-1 117 126.
DE-A-32 19 413 describes aqueous alkyd resin emulsions whose emulsified resin component consists of a mixture of an acrylate-modified air-drying alkyd resin having acid groups which are partially neutralized prior to emulsification in water and of a urethane-modified air-drying alkyd resin having tertiary amino groups.
DE-A-32 19 471 likewise embraces aqueous alkyd resin emulsions whose emulsified resin component consists of a mixture of acrylate-modified air-drying alkyd resin with partially neutralized acid groups and of a urethane-modified air-drying alkyd resin having tertiary amino groups, the acrylate-modified alkyd resin, unlike that employed in DE-A-32 19 413, additionally containing polyethylene glycol.
DE-A-33 15 690 describes a process for preparing air-drying aqueous emulsions of urethanemodified alkyd resins and/or urethane oils, where first of all a graft copolymer comprising unsaturated polyester, and acrylate monomers as emuslifier, are mixed with the air-drying urethane-modified alkyd resin and then the mixture obtained in such a manner is emulsified in water.
EP-A-0 267 562 describes a process for preparing water-thinnable air-drying paint binders on the basis of vinyl- and/or (meth)acrylic-modified alkyd resin emulsions, where a mixture of vinyl and/or (meth)acrylic monomers is polymerized to a degree of conversion of at least 95% in the presence of an aqueous solution or emulsion of an epoxy resin ester and/or urethane alkyd resin and/or alkyd resin which is soluble in water following at least partial neutralization, which have a content of fatty acids and which are grafted with methacrylic acid and with further monomers onto a portion of the unsaturated fatty acids, and in the presence of a free-radical initiator.
EP-A-0 305 795 embraces a process for preparing aqueous graft copolymer emulsions where, in a step (a), a polymer is prepared by esterifying unsaturated fatty acids, polyols having 3 to 6 hydroxyl groups and, if desired, polyethylene glycol and then graft-copolymerizing (meth)acrylic acid and nonfunctional, vinylically unsaturated monomers onto the unsaturated fatty acids, adding, in a step (b), water to said polymer following neutralization of the carboxyl groups of the (meth)acrylic acid with ammonia and/or organic amines, and, in a step (c) in this solution, polymerizing vinyl and/or (meth)acrylic monomers which essentially carry no functional group other than the ethylenically unsaturated double bond.
The above-listed laid-open publications all describe alkyd resin acrylate dispersions having a core-shell structure, where the core consists of a hydrophobic alkyd resin segment and the shell, which is grafted onto the core, consists of carboxyl-containing, neutralizable and thus hydrophilicizable acrylate segments. Storability is the principal advantage cited for this type of binder. The preparation of such polymers is highly complex (cf. in particular EP-A-0 267 562 and EP-A-0 305 795).